330 



abysso-pelagie and pelagic districts also occurs, as well as 

 between the abysso-pelao-ic and abyssal. 



The marine abyssal districts comprise the lightless depths, 

 or generally those depths exceeding one hundred fathoms. 

 As assimilating plant life is absent in these districts, the food 

 supply of the organisms existing in them must be wholly 

 derived from the districts in which such assimilating vegeta- 

 tion exists. A large proportion of the food of the abyssal 

 animals is contained in the organic oozes and sediments 

 which constantly settle down in a more or less decomposed 

 state from the lighted districts. The abysso-pelagie district 

 is frequently invaded by organisms from the pelagic district, 

 which descend into the dark regions during the day. 



Bio>:oMi(' CHARACTEiasTics OF Marlxi: Oroaxisms. 



Having now considered the life-districts, we will next turn 

 to a study of the organisms inhabiting them. A bionomic 

 classification of animals and plants cannot strictly agree 

 with a classification based on iiurely anatomical characteris- 

 tics. It is a more primitive classification; but from the 

 point of view here adojjted, it is the most convenient. The 

 following five groups, based on the habits and mode of loco- 

 motion of the organisms, will be found to be a convenient, 

 and, from the bionomic point of view, a natural classifica- 

 tion: Plankton, Xekton. Benthos, Mero-plankton, and 

 Pseudo-plankton . 



The term plankton was first introduced by Victor Hensen 

 in 1887. It was derived from the Greek TrAayKxos — mean- 

 ing to drift about aimlessly. In the marine i-ealm it com- 

 prises those organisms which spend their lives in the sea, 

 drifting aliout from place to place, without sufficient power 

 to direct their own course.* It comprises organisms ranging 

 from microscopic dimensions to meclusie 50 cm. in diameter. 

 (Walther, '97, p. 211. ) While some of the larger animals of 



* I follow Walther rather than Ortmann in the definition of the term plankton. The 

 latter employs it in a sense which I consider synonymous with pelagic as here defined. The 

 distinction between Plankton and Nekton seems an important one, even though inter- 

 mediate tj-pes are common. 



